1. Field of the Invention
The present invention relates to a vector control apparatus, and more particularly, to a sensorless vector control apparatus and method for controlling a variable speed operation and speed of an induction motor.
2. Description of the Background Art
In general, an induction motor has been primarily used for a constant speed operation as it is more difficult to control in comparison to a DC motor. However , as a vector control theory is introduced and a high performance central processing unit (CPU) or digital signal processor (DSP) is developed, the induction motor is now capable of being controlled for a variable speed operation.
The vector control theory is a method in which three phase AC powers (xe2x80x98axe2x80x99 phase, xe2x80x98bxe2x80x99 phase and xe2x80x98cxe2x80x99 phase) inputted at 120xc2x0 intervals are disassembled (converted) by a direct axis and a quadrature axis of 90xc2x0 intervals, and their size are controlled to a desired value and restored (inverted) to the three phase powers, to thereby control the three AC powers. This method is mainly used to control the induction motor.
In order to vector-control the induction motor, the speed or magnetic flux information of the induction motor is required. In order to measure the speed or the magnetic flux information, a speed sensor or a magnetic flux sensor such as a Tacho generator, resolver or a pulse encoder is required.
However, since the sensors include an electronic circuit, the induction motor having the sensors must be used within the operable temperature range of the electronic circuit, and signal wiring between the speed sensor and the inverter incurs much expense.
Also, the use of sensors is avoided as the connections between the sensors and the induction motor are susceptible to damage upon impact of the motor.
Accordingly, recently, various speed estimation methods of the induction motor has been proposed with respect to the sensorless vector control without the speed sensor. Among them, as a high speed algorithm method, a method based on a model reference adaptive system, an adaptive observer, that is, a method for estimating a speed or a slip frequency independently from a main control system, is used for consideration of a stability of a speed estimation, and as a low speed algorithm method, a high frequency injection method is used.
FIG. 1 is a schematic block diagram of a speed control apparatus which supplies a synchronous speed according to a voltage to frequency method in accordance with the prior art.
As shown in FIG. 1, the conventional speed control apparatus includes an angular velocity generator 1 receiving a command frequency (F) by a user""s input, converting it to an electric angular velocity (We) to be applied to a motor, and outputting it; a voltage generator 2 receiving the command frequency (F), generating a voltage (V) according to a Voltage to Frequency ratio(V/F ratio), and outputting it; and an inverter 3 controlling a speed of an induction motor (IM) by using the electric angular velocity (We) outputted from the angular velocity generator 1 and the voltage outputted from the voltage generator 2.
The operation of the conventional speed control apparatus constructed as described above will now be explained.
Generally, in an industrial site, the speed detecting unit is not required, and instead, a common inverter of a variable voltage variable frequency (VVVF) method which is simply controlled is widely used.
In order to constantly maintain a flux of the induction motor, the common inverter constantly controls a ratio between an output voltage of the inverter and an output frequency (V/F=constant), and a synchronous speed (rpm(rotation per minute)) of a rotational magnetic field is controlled by varying the output frequency.
Synchronous speed (rpm)=120*F/Pxe2x80x83xe2x80x83(1)
wherein xe2x80x98Pxe2x80x99 indicates the number of poles of a stator winding and xe2x80x98Fxe2x80x99 indicates a command frequency of a current flowing at the stator winding.
Input voltage (Vs) is determined as follows:
Vs=Rs*Is+(Lls+Lm)*dls/dtxe2x80x83xe2x80x83(2)
wherein xe2x80x98Rsxe2x80x99 indicates a stator resistance, xe2x80x98Isxe2x80x99 indicates an input current of the induction motorxe2x80x99, xe2x80x98Llsxe2x80x99 indicates stator leakage reactance, and xe2x80x98Lmxe2x80x99 indicates a magnetized reactance.
In the case where the stator resistance (Rs) of the inductor motor is not used in equation (2), the equation becomes:
Vs=(Lls+Lm)*dls/dtxe2x80x83xe2x80x83(3)
Generally, the stator leakage reactance (Lls) is relatively small compared to the magnetized reactance (Lm) in equation (3). Accordingly, equation (3) is computed by equation (4):
xe2x80x83Vs=Lm*dls/dt=We*Lm*ls=2xcfx80F*xcfx86xe2x80x83xe2x80x83(4)
In equation (4), since Vs/F=2xcfx80F*xcfx86, by constantly providing the ratio of Vs/F, the motor can be controlled while constantly maintaining the flux.
Accordingly, when the command frequency (F) is determined, it is converted to a synchronous speed (We=2xcfx80F) and applied to the induction motor. At this time, in order to constantly maintain the flux of the induction motor, a voltage is generated corresponding to the command frequency (F) so that the V/F ratio is constant, and outputted to the inverter.
Then, the inverter generates three phase voltages by using the synchronous speed (We) and a voltage and supplies them to the induction motor (IM). That is, if the ratio of the V/F is constantly provided, since the flux is constantly maintained, the induction motor can be controlled.
In this respect, since the induction motor is rotated at a slower speed than the synchronous speed, a slip is obtained by the following equation (5):
Slip=(Wexe2x88x92Wr)We
Wherein xe2x80x98Wexe2x80x99 indicates a synchronous speed and xe2x80x98Wrxe2x80x99 indicates a speed of the induction motor.
FIG. 2 is a graph showing a slip-torque curve wave form of a load and a motor according to the V/F method of the conventional art.
As shown in FIG. 2, the induction motor is operated at an intersection point of the load and the slip-torque curve of the induction motor, and a corresponding current flows.
FIG. 3 is a schematic block diagram of the vector control apparatus in accordance with the conventional art.
As shown in FIG. 3, a vector control apparatus having an inverter for receiving a speed command value (wr*) from a user and supplying three phase currents required for an induction motor, including: a first proportional integrator 5 for receiving an error between the speed command value (wr*) inputted from a user and a speed (wr) actually detected from an induction motor and generating a current command value of xe2x80x98qxe2x80x99 axis component (iqse*); a second proportional controller 8 for receiving an error, that is difference between the current command value of xe2x80x98qxe2x80x99 axis (iqse*) according to a rating of a motor and an actual current of xe2x80x98qxe2x80x99 axis (iqse) flowing through the motor, and generating and outputting a voltage (vqse) for operating the motor at the speed command value (wr*); a third proportional integrator 9 for receiving an error between a current command value of xe2x80x98dxe2x80x99 axis component (idse*) according to a rating of the motor and an actual current of xe2x80x98dxe2x80x99 axis (idse) flowing at the motor, and generating and outputting a voltage (vdse) for operating the motor at the speed command; a static coordinate system converter 10 for receiving the two phase voltages (vqse and vdse) and outputting three phase voltages Va, Vb and Vc; a synchronous coordinate system converter 12 for measuring three phase currents (ias, ibs and ics) inputted to the induction motor, changing the actual current (idse) of xe2x80x98dxe2x80x99 axis flowing at the motor and the actual current (iqse) of xe2x80x98qxe2x80x99 axis flowing at the motor, and outputting it; a slip frequency generator 13 for receiving a current command value of xe2x80x98qxe2x80x99 axis component (iqse*) and a current command value of xe2x80x98dxe2x80x99 axis component (idse*) and generating a slip frequency; an arithmetic control signal generator 14 for receiving a slip frequency (Wslip) of the slip frequency generator 13 and the actually detected speed (Wr) of the induction motor and generating an angular velocity (We); an inverter 11 for receiving the three phase voltages Va, Vb and Vc and controlling a speed of the induction motor; and a speed sensor 15 connected to a shaft of the induction motor, for detecting a speed of the induction motor.
The operation of the conventional vector controlling apparatus constructed as described above will now be explained.
First, a first operator obtains an error between thee speed command value (Wr*) and the actually detected speed (Wr) and provides it to the first proportional integrator 5, the first proportional integrator 5 creates a current command value of xe2x80x98qxe2x80x99 axis component (iqse*) (a torque component) and provides it to a non-inverting terminal (+) of a second operator 6, and a current command value of xe2x80x98dxe2x80x99 axis component generated according to the rating of the motor (idse*) is provided to the non-inverting terminal (+) of a third operator 7.
Then, the second operator 6 and the third operator 7 obtain an error between the xe2x80x98dxe2x80x99 and xe2x80x98qxe2x80x99 axis current command values (idse* and iqse*) and the xe2x80x98dxe2x80x99 and xe2x80x98qxe2x80x99 axis actual currents (idse and iqse) and provide the error to the second proportional integrator 8 and the third proportional integrator 9.
And then, second and third proportional integrators 8 and 9 generate xe2x80x98dxe2x80x99 axis and xe2x80x98qxe2x80x99 axis voltages vdse and vqse, and transmit them to the static coordinate system controller 10, respectively.
At this time, the slip frequency generator 13 obtains Wslip by using xe2x80x98dxe2x80x99 and xe2x80x98qxe2x80x99 axis current command values (idse* and iqse*) and provides it to the non-inverting terminal (+) of the arithmetic control signal generator 14.
The speed sensor 15 for sensing a speed of the induction motor IM provides the sensed motor speed (Wr) to another non-inverting terminal (+) of the arithmetic control signal generator 14.
Then, the arithmetic control signal generator 14 obtains a slip frequency (Wslip) and the motor speed (Wr), computes a synchronous angular velocity (We) of the induction motor, generates an arithmetic control signal for converting a two phase voltages to three phase voltages or three phase voltages to two phase voltages by using the computed synchronous angular velocity (We), and provides it to the static reference frame converter 10 and the synchronous reference frame converter 12.
Accordingly, when the stationary reference frame converter 10 converts the two phase voltages vqse and vdse into three phase voltages va, vb and vc and provides them to the inverter 11, the inverter 11 receives the three phase powers and drives the induction motor.
The sensorless vector control method uses a speed estimation algorithm instead of a speed sensor which detects a speed of the induction motor.
Meanwhile, the conventional variable voltage variable frequency (V.V.V.F) method which does not require a speed detecting apparatus and simply performs a speed control is a V/F method for constantly maintaining the flux of the induction motor simply.
The vector control method of the conventional art, however, has several disadvantages.
That is, first, even though the speed command value is provided as a frequency (F), the value provided to the induction motor is a synchronous speed (We), the actual motor speed is given as xe2x80x98Wr=We(1-slip)xe2x80x99, causing a problem that the actual motor speed is varied by the slip frequency which is changed according to a load.
Secondly, though the ratio of the V/F is constantly maintained to constantly control the flux, since the stator resistance (Rs) contained in the voltage equation (2) is not used, so that the flux is supplied less at a low speed, resulting in a problem that the motor fails to generate a desired output torque.
Thirdly, the conventional vector control method is that, in controlling a speed of the motor with a low speed algorithm, an absolute position of the magnetic flux of the motor is searched by adding a high frequency voltage or a current to a basic wave voltage. This method is thus, effective at a low speed, but not used at a high speed. Conversely, in case of controlling a speed of the motor with a high speed algorithm, the method is effective at a high speed but implementation of the algorithm itself at a low speed is very difficult, causing a problem that it fails to be used to cover the whole velocity range.
Fourthly, in order to use the high speed and low speed algorithm, a parameter of a motor should be accurately obtained. If the parameter is not accurate, a system is unstable, for which, thus, numerous arithmetic operations are required for implementation of algorithm. For this purpose, a central processing unit (CPU) or a digital signal processor (DSP) of a high performance is required, which is difficult to be adopted in general.
Therefore, an object of the present invention is to provide a vector control apparatus that is capable of preventing a speed variation according to a load and preventing a torque reduction according to a reduction of a flux at a low speed, thereby solving a problem of an initial torque reduction.
Another object of the present invention is to provide a vector control apparatus that is capable of performing a sensorless vector control over the whole velocity range.
Still another object of the present invention is to provide an easily realizable algorithm without using a high performance main operating unit by reducing a dependency on an induction motor parameter and operation amount.
Yet another object of the present invention is to provide a vector control apparatus that is capable of allowing a system to be operated in a stable region in any circumstances by solving a problem of the conventional system operated in an unstable region on a speed-torque curve.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a sensorless vector control apparatus including: a speed controller for receiving a speed command value (Wr*) from a user and outputting a synchronous speed (We); a xe2x80x98dxe2x80x99 axis voltage command unit for receiving an actual current (ids) of xe2x80x98dxe2x80x99 axis and a current (iqs) of xe2x80x98qxe2x80x99 axis flowing through a motor and generating a xe2x80x98dxe2x80x99 axis voltage (Vds); a xe2x80x98qxe2x80x99 axis voltage command unit for receiving the synchronous speed (We) from the speed controller, receiving a current command value of xe2x80x98dxe2x80x99 axis component (ids*) according to a rating of the motor, and generating a xe2x80x98qxe2x80x99 axis voltage (Vqs); a voltage converter for receiving the xe2x80x98qxe2x80x99 axis voltage (Vqs) from the xe2x80x98qxe2x80x99 axis voltage command unit and the xe2x80x98dxe2x80x99 axis voltage (Vds) from the xe2x80x98dxe2x80x99 axis voltage command unit, and converting the two phase voltages (Vqs and Vds) to three phases (Va, Vb, Vc); and an inverter for receiving the three phase powers from the voltage converter and controlling a speed of the induction motor.
To achieve the above objects, there is further provided a sensorless vector control method for receiving the speed command value from a user and controlling the speed of the induction motor, including the steps of: receiving the speed command value (Wr*) by the speed controller; controlling the speed by using the speed controller to compensate a speed variation according to a load variation; operating the synchronous speed (We) outputted from the speed controller and the current command value (ids*) of a flux component and the actual current of xe2x80x98dxe2x80x99 axis (ids) and the actual current of xe2x80x98qxe2x80x99 axis (iqs), and generating a xe2x80x98qxe2x80x99 axis voltage (Vqs) and a xe2x80x98dxe2x80x99 axis voltage (Vds); three-phase converting the generated xe2x80x98qxe2x80x99 axis voltage and xe2x80x98dxe2x80x99 axis voltage, and supplying the three phase powers to the inverter.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.